This invention relates to electronic triggering circuits, and more particularly to triggering circuits responsive to the occurrence of a limited amplitude input signal, sometimes referred to as a "runt". A typical example of a limited amplitude signal is a pulse that switches from a valid logic state to an invalid logic state, and then switches back to the original valid logic state.
Trigger circuits are well known in the art for starting the sweep of an oscilloscope trace across a display device such as a cathode ray tube ("CRT") at the same trigger point on a repetitive signal. Typically, prior art trigger circuits include slope and level controls. The slope control determines whether the trigger point is found on the rising or the falling edge of the repetitive signal. The level control is a single voltage threshold level that determines where on that edge the trigger point occurs.
A block diagram of a typical oscilloscope 10 is shown in FIG. 1. An input signal is received at input terminal 12 and amplified by a preamplifier 14. The output of the preamplifier 14 is further amplified by vertical amplifier 16 to control the amplitude of the trace of the input signal on CRT 18. The output of the preamplifier 14 is also coupled to the input 22 of trigger circuit 20. The output 24 of trigger circuit provides a trigger pulse to activate timebase circuit 26. Timebase circuit 26 is typically a ramp signal for real-time analog oscilloscopes, and a clocked memory for digital storage oscilloscopes. The output of the timebase circuit 26 is amplified by horizontal amplifier 28 and controls the rate at which the trace of the input signal is swept across CRT 18.
Single threshold trigger circuits are not well suited to detecting limited amplitude signals. Digital signals, for example, have a lower threshold and an upper threshold that define valid logic low and high states, as well as an invalid or forbidden zone. A single threshold cannot completely define a digital signal, and therefore cannot provide the basis for a trigger circuit capable of triggering on the occurrence of a limited amplitude signal that switches from a valid logic state to an invalid logic state and back to the original valid logic state.
A trigger circuit having two voltage thresholds or level controls is shown in U.S. Pat. No. 4,823,076 to Haines et al. FIG. 4 of Haines et al shows a decoder 815 that provides signals VH, VT, and VL that are each components of an input digital signal indicating when the input signal is in the logic high, transition (invalid or forbidden), and logic low zones. These signals, in addition with considerable additional hardware and software, provide a trigger circuit that is capable of triggering on any pattern of logic high, low, or invalid states in an input signal.
What is desired is a hardware trigger circuit having a relatively low parts count that is capable of triggering on the occurrence of a limited amplitude input signal.